Identification of dominant antigens recognized by sera of sheep and mice experimentally infected with Coxiella burnetii

Abstract

Q fever is a disease caused by the rickettsial organism Coxiella burnetii that affects mostly humans, cattle, sheep, and goats. Little is known about the epitopes of the organism that are recognized by the host after infection. To determine the immunodominant antigens during infection with C. burnetii, two experimental animal models were studied. Mice were infected intraperitonally with a sub-lethal challenge dose of phase I, Nine Mile strain of C. burnetii (RSA439) and antibody response was monitored by Western immunoblot analysis at weeks 0, 2, 3 and 5 post-infection. Groups of adult sheep were challenged nasally and subcutaneously at post-breeding day 65, 100, or 135 and antibody response was monitored from the time of inoculation to the time of post-lambing or abortion and for 6 weeks thereafter. Serum from mice recognized ~14, ~20, ~28, ~32, and ~60 kDa C. burnetii proteins as immunodominant antigens. Prior to challenge, seronegative (negative phase I and phase II indirect immunofluorescence assays) adult sheep had significant cross-reactive antibodies to a variety of C. burnetii antigens but some developed a new, strong post-challenge response to a ~28 kDa moiety. C. burnetii-specific antibodies tended to appear earlier in mice, but this may be related to inoculum dose. Identification and characterization of immunodominant antigens, especially the ~28 kDa species, will provide essential information for the development of novel, highly specific diagnostic reagents and potential subunit vaccine candidates. Many aspects of C. burnetii replication and pathogenesis are still poorly understood. Because the organism can potentially be used as a weapon in bioterrorist activities, there is an imminent need to further characterize C. burnetii.